The Digestive System

In this article

Food's Journey Through the Digestive System continued...

While food is in the small intestine, nutrients are absorbed through the walls and into the bloodstream. What's leftover (the waste) moves into the large intestine (large bowel or colon).

Everything above the large intestine is called the upper GI tract. Everything below is the lower GI tract

Stop 4: The Colon, Rectum, and Anus

The colon (large intestine) is a five- to seven -foot -long muscular tube that connects the small intestine to the rectum. It is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon and the sigmoid colon, which connects to the rectum. The appendix is a small tube attached to the ascending colon. The large intestine is a highly specialized organ that is responsible for processing waste so that defecation (excretion of waste) is easy and convenient.

Stool, or waste left over from the digestive process, passes through the colon by means of peristalsis, first in a liquid state and ultimately in solid form. As stool passes through the colon, any remaining water is absorbed. Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it into the rectum, usually once or twice a day.

It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool, it empties its contents into the rectum to begin the process of elimination.

The rectum is an eight-inch chamber that connects the colon to the anus. The rectum:

Receives stool from the colon

Lets the person know there is stool to be evacuated

Holds the stool until evacuation happens

When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates so that the sensation temporarily goes away.